High in specific strength and specific rigidity, fiber-reinforced composite materials are useful and have been used in a wide variety of applications including aircraft structure members, windmill blades, automobiles' exterior plates, and computer parts such as IC trays and laptop enclosures (housing), and demands for them have been increasing every year.
A fiber-reinforced composite material has a heterogeneous structure produced by molding a prepreg consisting essentially of reinforcement fiber such as carbon fiber and a matrix resin, and accordingly, such a structure has large differences in physical properties between the alignment direction of the reinforcement fiber and other directions. For instance, it is known that the interlaminar toughness, which represents the resistance to interlaminar fracture of the reinforcement fiber layers, cannot be improved drastically by simply increasing the strength of the reinforcement fiber. In particular, fiber-reinforced composite materials containing a thermosetting resin as matrix resin are generally liable to be fractured easily by a stress caused in a direction other than the alignment direction of the reinforcement fiber, reflecting the low toughness of the matrix resin. In this respect, various techniques have been proposed aiming to provide composite materials that have improved physical properties, including interlaminar toughness, to resist a stress in directions other than the alignment direction of the reinforcement fibers while maintaining high compressive strength in the fiber direction under high temperature and high humidity conditions, which is required for manufacturing aircraft structural members.
Furthermore, fiber-reinforced composite materials have recently been applied to an increased range of aircraft structural members, and fiber-reinforced composite materials are also in wider use for windmill blades and various turbines designed to achieving improved power generation efficiency and energy conversion efficiency. Studies have been made to provide thick members produced from prepreg sheets consisting of an increased number of layers as well as members having three-dimensionally curved surfaces. If such a thick member or curved-surfaced member suffers from a load, i.e., tensile or compression stress, the prepreg fiber layers may receive a peeling stress generated in an antiplane direction, which can cause opening-mode interlayer cracks. As these cracks propagate, the overall strength and rigidity of the member can deteriorate, possibly leading to destruction of the entire member. Opening-mode, that is, Mode I, interlaminar toughness is necessary to resist this stress.
Compared to this, there is a proposal of a technique that uses high-toughness particle material of, for example, polyamide provided in regions between fiber layers so that the Mode II interlaminar toughness will be increased to prevent damage to the surface that may be caused in falling weight impact test (see patent document 1).
Aside from this, another document has disclosed a material that contains polyamide fine particles including specific polyamide combined with a thermosetting resin and have high strength against interlaminar shearing under wet heat conditions in addition to impact resistance (see patent document 2). Another document has disclosed thermoplastic resin particles in which a thermoplastic resin insoluble in a matrix resin is melt-blended with a thermoplastic resin soluble in the matrix resin (see patent document 3).
Aside from this, there is known a technique in which specific thermoplastic resin particles are combined with a matrix resin containing an elastomer as a technique aimed at improving Mode II interlaminar toughness (patent document 4).